The present invention relates to a device for heating a sleeve which when heated can shrink, for example to produce such a shrinking, when the sleeve is applied to reinforce a splice region between optical fibers or optical ribbon fibers or similar products.
Sleeves which are shrinkable by heat are often used to protect joints between electrically conducting wires and cables and between optical fibers, for example for individual fibers or ribbon fibers used in the telecommunication technology. When used, such a sleeve is applied over an end of a line, e.g. a ribbon fiber, thereafter the line is spliced to another line, the shrinkable sleeve is placed over the splice and finally the shrinkable sleeve is heated making it shrink and firmly grip around the lines, in particular around the splice region and over adjacent isolation or surface layers for signal conductors. In the heating it must be carefully observed that no air is left inside the shrinkable sleeve, since it will necessarily degrade the mechanical and protecting function of the sleeve.
For the heating usually an electrical resistance element is used, the wires included therein located close to the place, where the shrinkable sleeve is placed before the shrinking operation. Then it is a problem to mount the resistance element in such a way that it encloses the sleeve at several sides thereof to allow a rapid and efficient heating, and at the same time it should be possible to easily introduce cooling air in the space where the heating is made.
An optical fiber intended for telecommunication normally consists of a light conductor of quartz glass and an enclosing protective sleeve of a plastics material protecting the optical fiber against mechanical and chemical influence. A ribbon fiber or a fiber ribbon consists of a plurality of such optical fibers placed in parallel in a plane at the sides of each other, attached to each other by a polymer layer or polymer sleeve, so that the optical fibers form a ribbon of fibers enclosed by a flat sleeve, also called the secondary protection of the fibers. When an optical fiber or a ribbon fiber is to be welded to other fibers, firstly the various protecting sleeves around the very fibers made of glass must be removed. Thereafter the optical fiber or the different fibers in a ribbon fiber can be welded to another optical fiber or to several optical fibers possibly included in a different ribbon fiber. At the welding position the fibers made of glass are bare or naked and this portion of the fibers must then again be protected after executing the welding. It is often accomplished by placing a shrinkable sleeve over one of the fibers or over a ribbon fiber before the welding operation, and by then placing this shrinkable sleeve, after the welding has been made, over the very welding position. The spliced fibers or ribbon fibers are together with the sleeve then transported to a place, where the shrinkable sleeve is heated to such a high temperature, that it melts or shrinks and thereby protects the optical fiber joint or the optical fibers in the joint.
Presently used ovens are heated by a heating element, for example a resistive heating wire, heating a U-shaped cradle that in turn heats the sleeve. Over the cradle a lid is closed during the melting or shrinking step. Such a system results in that the heating or shrinking process has a slow control since also the cradle must be heated. The sleeve is heated to such a temperature that it starts collapsing, and then it is important that it collapses from the center and outwards from that place, towards the two sides starting from the center, so that no air bubbles can be formed inside the shrunken sleeve. When the whole sleeve has collapsed, the heating element is switched off and the element, cradle and sleeve and optical fibers are allowed to cool, until the sleeve has become more rigid and has cooled so much, that it can be handled. The cooling process normally takes a fairly long time, since both the sleeve and the cradle must be allowed to cool, and furthermore they are to some extent enclosed by other elements making the cooling not very efficient. The splice together with the applied sleeve is therefor lifted out of the oven, after the lid over the cradle has been opened, and is allowed to cool for an additional time period outside the oven before it can handled again. When using such methods often the final cooling of the protective sleeve is the time limiting portion of the welding process, the other steps of which are executed rather quickly, and this can be experienced as disturbing by many users.
U.S. Pat. Nos. 5,434,387 and 5,030,810 disclose a device for melting shrinkable sleeves on for example optical fibers. The device comprises a number of independently heatable heating elementary parts attached to the interior side of cylindrical wall entirely surrounding the fiber. A substantially free suspension of the fiber can be obtained.
The published European patent application EP-A1 0 096 550 schematically describes apparatus for melting shrinkable sleeves onto optical fibers, wherein a single glow wire can be designed to achieve shrinking from the center and outwards in order to avoid trapped air bubbles.
U.S. Pat. No. 4,460,820 discloses a device for melting shrinkable sleeves onto optical fibers, comprising a number of independently heatable heating elements having resistance wire enclosed therein, which project from a base plate upwards, towards the fiber. The fiber is placed freely suspended between support blocks.
It is an object of the invention to provide a resistance element for a heating device for shrinkable sleeves, which can be easily heated and be rapidly cooled.
It is another object of the invention to provide a resistance element for a heating device for shrinkable sleeves, which has a simple mounting attachment and can be easily electrically contacted.
The problem solved by the invention is to produce a rapid heating and a rapid cooling of a shrinkable sleeve, placed around for example an optical fiber or an optical ribbon fiber. This problem is solved by using a heating element having an open structure, having a minimum thermal capacity and/or minimum heat conducting areas to the surrounding device in which it is placed and/or at least partly surrounding the shrinkable sleeve to be heated.
Thus generally, a resistance element for a device for shrinking shrinkable sleeves can comprise a thin, elongate electrical conductor or wire extending in a zigzag path and bent to a tunnel shape. The shrinkable sleeve is intended to be placed in the region of the resistance element that corresponds to approximately the geometric axis of the upper semi-cylindrical portion of the tunnel shape. The mounting of the resistance element is only made by a few thin pins, located at the front and rear sides of the element and intended to be placed in corresponding recesses in a lid in the device, most of the pins being connected to the conductor extending in zigzag through narrow connecting portions. In addition, the rear pins have portions intended for contact with electrically conducting, elastic contact pins, with which the resistance element comes in contact, when the lid is folded down to execute the very heating and shrinking operation. The open and freely suspended design of the resistance element having only small contact areas to the lid results in that the resistance element can be rapidly heated and thereafter be rapidly cooled implying the same conditions to be true also for the shrinkable sleeve placed inside the resistance element and also having the effect that a shrinking operation can be executed within a minimum time period.
In its most general form a heating device for shrinkable sleeves thus comprises a heating resistance element having an elongate shape, which at least partly encloses a shrinkable sleeve places therein, so that the longitudinal direction of the element is parallel to the axis of the shrinkable sleeve. The heating element is attached to the other parts of the device at only a few, localized areas, which together, i.e. the added area of which, constitute a very little part of the total area of the heating element. The heating element is advantageously made as a single part made from electrically conducting material. Then it has conductor portions extending forwards and backwards, substantially perpendicularly to the direction of a fiber placed therein. The conductor path of the element can then be described such as that it for example substantially has a zigzag shape. Moreover, the element can be bent to have a form like a tunnel. The form can comprise a portion having the shape of a cylindrical shell which at least partly encloses a fiber placed therein.